Excimer lasers are currently becoming the workhorse light source for the integrated circuit lithography industry. A typical prior art KrF excimer laser is depicted in FIG. 1 and FIG. 9. A pulse power module 2 provides electrical pulses lasting about 100 ns to electrodes 6 located in a discharge chamber 8. The electrodes are about 28 inches long and are spaced apart about 3/5 inch. Typical lithography lasers operated at a high pulse rate of about 1,000 Hz to 4,000 Hz. For this reason it is necessary to circulate a laser gas (about 0.1 percent fluorine, 1.3 percent krypton and the rest neon which functions as a buffer gas) through the space between the electrodes. This is done with tangential blower 10 located in the laser discharge chamber. The laser gasses are cooled with a heat exchanger also located in the chamber. Commercial excimer laser systems are typically comprised of several modules that may be replaced quickly without disturbing the rest of the system. Principal modules are shown in FIG. 1 and include:
Laser Chamber 8, PA1 Pulse Power Module 2, consisting of three submodules PA1 Output coupler 16, PA1 Line Narrowing Module 18 PA1 Wavemeter 20 PA1 Computer Control Unit 22 PA1 Peripheral Support Sub systems PA1 Blower 10
The discharge chamber is operated at a pressure of about three atmospheres. These lasers operate in a pulse mode at about 600 Hz to about 1,000 Hz, the energy per pulse being about 10 mJ and the duration of the laser pulses is about 15 ns. Thus, the average power of the laser beam is about 6 to 10 Watts and the average power of the pulses is in the range of about 700 KW. A typical mode of operation is referred to as the "burst mode" of operation. In this mode, the laser produces "bursts" of about 50 to 150 pulses at the rate of 1,000 pulses per second. Thus, the duration of the burst is about 50 to 150 milliseconds. Prior art lithograph, excimer lasers are equipped with a feedback voltage control circuit which measures output pulse energy and automatically adjusts the discharge voltage to maintain a desired (usually constant) output pulse energy. It is very important that the output pulse energy be accurately controlled to the desired level.
It is well known that at wavelengths below 300 nm there is only one suitable optical material generally available for building the stepper lens used for chip lithography. This material is fused silica. An all fused silica stepper lens will have no chromatic correction capability. The KrF excimer laser has a natural bandwidth of approximately 300 pm (full width half maximum). For a refractive system (with NA&gt;0.5)--either a stepper or a scanner--this bandwidth has to be reduced to below 1 pm. Current prior art commercially available laser systems can provide KrF laser beams at a nominal wavelength of about 248 nm with a bandwidth of about 0.8 pm (0.0008 nm). Wavelength stability on the best commercial lasers is about 0.25 pm. With these parameters stepper makers can provide stepper equipment to provide integrated circuit resolutions of about 0.3 microns. To improve resolution a narrower bandwidth is required. For example, a reduction of a bandwidth to below 0.6 pm would permit improvement of the resolution to below 0.25 microns.
Argon fluoride, ArF excimer lasers which operate at a wavelength of about 193 nm using a gas mixture of about 0.08 to 0.12% fluorine, 3.5% argon and the rest neon, are beginning to be used for integrated circuit lithography. F.sub.2 lasers produce laser radiation at wavelengths of about 159 nm. The gas mixture typically is 0.1% percent fluorine and the rest helium or neon.
Gas discharge laser typically use a preionizer technique for preionizing the gas between the electrodes prior to the main electrical discharge. Examples of these preionizers are spark gap preionizers and corono discharge preionizers. Spark gaps produce ions with a discharge between two electrodes like an automatic spark plug. A corono discharge preionizer produce ions by creating a corono of ions adjacent to a conductor at high voltage. A typical corono discharge preionizer is described in U.S. Pat. No. 5,337,330 which is incorporated herein by reference. The ionization produced by these preionizers produces ultraviolet radiation which in turn reacts with the laser gas to generate a substantial ion population in the laser gas between the electrodes. Typically spark gap preionizers produces higher energy ultraviolet radiation than corono discharge preionizers, but the radiation from the corono discharge preionizers tends to be much more uniform.
It is known that the addition of about 10 to 50 ppm of oxygen to an excimer laser gas mixture can be used to stabilize the efficiency and performance of the laser. These additives improve the preionization efficiency of the laser. See, for example, U.S. Pat. No. 5,307,364. Small quantities of xenon have been proposed as a gas additive for CO.sub.2 lasers. See Japan Patent Number JP 60180185 issued in 1984 based on a patent application filed on Feb. 27, 1984. In a 1995 article entitled, Tranmission Properties of Spark Preionization Radiation in Rare-Gas Halide Laser Gas Mixes, (IEEE Journal of Quantum Electronics, Vol. 31, No. 12, December 1995), the author discusses gas additives to enhance preionization of rare-gas halide lasers. This paper deals with lasers utilizing spark gap preionization. Spark gap preionization is known to produce high energy photons which in turn preionizes laser gas between the laser electrodes. The author points out that the ionization potential of xenon is too high (i.e., greater than the preferred ionization potential of &lt;10 ev); however, the author suggests that it might be possible to use small quantities of xenon in lasers which have transmission windows at vacuum ultraviolet wavelengths &lt;103 nm or photon energies in excess the 12.1 ionization potential of xenon. The article infers that spark gap photons have energies less than 10 ev and suggests that higher energy photons such as x-rays could be used to excite xenon if used as an additive.
The actual performance of integrated circuit lithography equipment then depends critically on maintaining minimum bandwidth of the laser throughout its operational lifetime, and also minimizing the laser's energy variation from pulse-to-pulse.
Therefore, a need exists for a reliable, production quality excimer laser system, capable of long-term factory operation and having accurately controlled pulse energy stability, wavelength, and a bandwidth.